HLA class II-restricted CD4+ T cells play a critical role in cellular immunity and are a key component of anti-tumor immune responses. CD4+ T cells provide necessary help to tumor-specific CTLs (Topalian 1994. Curr Opin Immunol 6:741-745) and produce cytokines such as interferon gamma (IFNγ), which can activate antigen presenting cells and mediate other immunological effects (Corthay et al., 2005. Immunity 22:371-383). Experimental results in several systems have demonstrated that CD4+ T cells are necessary for an effective anti-tumor immune response. Given the importance of CD4+ T cells in generating a robust immune response, an optimally designed cancer immunotherapy or anti-tumor vaccine should induce both tumor-specific CD4+ and CD8+ T cells for maximal efficacy.
The human prostatic acid phosphatase (PAP) is a phosphatase predominantly expressed in the prostate gland. Elevated serum level of PAP is often observed in patients with prostate cancer or other prostate conditions, with the highest levels of PAP found in metastasized prostate cancer. In addition, diseases of the bone, such as Paget's disease or hyperparathyroidism, diseases of blood cells, such as sickle-cell disease, multiple myeloma, or lysosomal storage diseases, such as Gaucher's disease, will show moderately increased levels of PAP. Because over 95% of prostate cancer cells express PAP, several immunotherapeutic strategies for prostate cancer have been devised using PAP as a target. For instance, Cancer Biology and Therapy (March 2005, vol. 4, issue 3) has reported promising results from a clinical study in which a patient's own immune cells were collected, stimulated to become immunoreactive to PAP, and then returned to the patient by intravenous injection. These new immunological approaches rely on methods that can effectively induce a PAP-specific immunity, including T cell-mediated immunity.
The usefulness of a defined T cell epitope is limited by its HLA-restriction. Peptide epitopes typically form productive peptide-MHC complexes with a small number of HLA alleles and stimulate T cell responses only in individuals expressing those alleles. This confines immunological studies and clinical trials to individuals of a specific HLA type, often 20% or less of the general population. So-called promiscuous T cell epitopes, which can be presented by a larger number of HLA alleles, have been described for several tumor antigens. Promiscuous T cell epitopes can bind to multiple HLA alleles to stimulate antigen-specific T cells, allowing for the induction and study of T cell responses in individuals of different HLA types. Additionally, promiscuous epitopes are valuable because the immunotherapies and vaccines based on these epitopes can be widely applicable to the general population for cancer treatment and prevention. Thus, there exists a clear need for new information relating to previously unknown promiscuous epitopes of tumor antigens, including PAP.
The present inventors have identified a series of novel promiscuous T cell epitopes in the human PAP protein sequence. These epitopes, comprising the region of 257-271 of the PAP protein, are recognized by a CD4+ T cell clone (PAPc66) in the context of HLA-DR and capable of inducing T cell activation when presented by antigen presenting cell of at least 15 different HLA-DRβ1* alleles. The promiscuity of these epitopes for different HLA-DRβ1* alleles makes these epitopes a valuable tool for evaluating PAP-specific immune responses regardless of the patient's HLA type. Additionally, these epitopes can be used as a universal CD4 T helper cell epitope in peptide-based vaccines or immunotherapies for the treatment PAP+ prostate cancers.